1. The Field of the Invention
The present invention generally relates to irreversible warmup indicators which, once warmed, indicate that a certain temperature has been exceeded even though the apparatus may later be recooled.
2. Description of the Prior Art
The prior art discloses many types of irreversible warmup indicators, but most are based on the fact that water expands as it is cooled from 4.degree. C. to 0.degree. C. When water is inclosed in a frangible container which is sealed, freezing of the water causes the container to break. Any increase in temperature causes the frozen water to thaw and indicate by wetting appropriately located filter paper. Such a device is exemplified by U.S. Pat. No. 3,786,777.
Similar devices make use of the fact that water flows at 0.degree. C. or higher. For example, U.S. Pat. No. 2,560,537 discloses a strip of paper which is encased in a transparent tube with a crimp in its center. Water is placed in one section of the tube and the device is activated by freezing. Any thawing of the device causes the water to liquify and thereby flow by diffusion through the paper, producing an indication.